cmdserver, runcommand: properly handle the client sending no arguments

No real reason for a client to do this, but still possible.

Previously if the client sent no arguments, a list with an empty string ['']
would be used as the arguments to dispatch, which would cause hg to complain
about an ambiguous command.

Instead, we simply check for no arguments and use an empty list instead (which
is equivalent to invoking hg with no args on the command line).

mercurial/commandserver.py

@@ -171,6 +171,9 class server(object):
         and writes the return code to the result channel """
 
         length = struct.unpack('>I', self._read(4))[0]
+        if not length:
+            args = []
+        else:
         args = self._read(length).split('\0')
 
         # copy the ui so changes to it don't persist between requests